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Treatment of [ClCH2Co^III(L1NH-py)(HL1NH-py)]ClO4, where HL1NH-py is the tridentate 2-((2-pyridylmethyl)amino)-3-butanone oxime and L1NH-py^- its conjugated base with diluted NaOH, afforded the new complex [CH2Co^III(L1NH-py)(HL1N-py)]ClO4, containing a three-membered metallacycle by a pathway involving the intramolecular nucleophilic addition of an equatorial nitrogen donor to the axial carbon. The X-ray analysis revealed a highly distorted structure with the distortion concentrated in the Co−C bond. The kinetics of formation of this complex and of the analogous complexes (previously synthesized) containing the 2-((2-(2-pyridyl)ethyl)amino)-3-butanone oxime ligand (HLNH-py) were studied. These studies revealed that the intramolecular process forming the metallacycle is quite rapid, whereas the carbon atom bonded to cobalt is inert toward nucleophilic intermolecular attack. The rapidity of the reaction was ascribed to a favorable entropy contribution and to the “proximity effect” imposed by the relatively rigid framework of the molecule, which holds together the reactive centers (C and N atoms) within a critical distance.

The complex [C6H5CH2Co(L)(LR)]ClO4 (3), where L = 2-((2-pyridylethyl)imino)-3-butanone oximato and LR = 2-((2-pyridylethyl)amino)-3-aminobutane, has been prepared by reduction with NaBH4 of the complex [Co^III(L)2]ClO4, followed by addition of C6H5CH2Cl. The compounds was characterized in the solid state by X-ray analysis. The complex undergoes Co−C homolytic cleavage under acidic conditions, giving dibenzyl under a nitrogen atmosphere and benzaldehyde in the presence of air.

THE reversible reaction of certain cobalt chelates with molecular oxygen can be considered as a simple model of the reversible oxygen binding by haemoglobin and myoglobin1,2. It was recently pointed out3–6that in suitable experimental conditions synthetic oxygen carriers show a reaction stoichiometry corresponding to one oxygen molecule per metal atom. In both cases the six-coordinated complex with oxygen can be formed by an equatorial tetradentate chelating agent and a donor ligand in the axial position trans to oxygen4. The reversibility of oxygenation seems to depend on a non-polar environment of the metal provided by the aprotic solvents in the simple complexes and by the hydrophobic pocket accommodating the haem in haemoglobin7.

Rate constants for the reaction of (I) with HSO3‐in aqueous solution are determined at 35 °C. Both electronic and steric effects concur to control the trans effect of the phosphine ligands in the title reaction.

The parallel lability trend for the title reactions of (I) and analogous Co compounds suggests a dissociative activation process for the reactions of the Rh complexes.